In marine exploration, seismic and various underwater applications, it is often necessary to measure, record and transmit to remote locations sampled readings of water depth and compass heading over a period of time. Acquisition of data representative of direction and depth has been achieved through the use of remote sensors which provide electrical signals responsive to magnetic compass heading and depth. In prior art systems, data representative of either compass heading or depth is transmitted to remote data acquisition units over a multiwire communication link. The communication link is typically disposed within a streamer, a cable normally employed to transmit between 64 and 1000 channels of hydrophone data to a remote data acquisition unit. The streamer is typically towed behind a vessel, thereby providing data from sensors at a number of sensor positions along the streamer.
It is well known in the art to sense compass heading using a submersible sensor. A patent on a remote reading compass has been granted to this applicant and assigned to the same assignee as the present application. Communication of data from remote transducers via a single two wire conductor has also been previously described in U.S. Pat. No. 4,093,946 of the same assignee.
In a typical seismic streamer or towed sonar array, a plurality of sensors, usually hydrophones, are connected to the streamer. In previous systems, sensors have been individually housed with separate sensor electronics permitting unique address selection and activation of the sensor responsive to address codes transmitted over the communication link by a data acquisition unit. Upon address recognition and selection of a specific sensor along the streamer, the selected sensor interface circuitry responds over the link with an address acknowledgement and subsequently with data representative of the compass heading or depth of the respective sensor. Acquisition of data from a plurality of sensors requires sequential polled transactions over the communications link.
Several limitations result from the independent housing of compass heading and depth sensors as suggested by the prior art. One limitation obtains from the requirement that each sensor be uniquely addressed or polled over the link. A typical data transmission period including address transmission address acknowlegement and data response intervals in total may occupy 40 milliseconds. Thus, the system is bandwidth limited to the inverse of the data transmission period, to approximately 25 samples per second. It is therefore apparent that repetition of the address interrogation sequence when addressing a plurality of sensors limits the achievable system data sampling rate. Moreover, housing compass heading and depth sensors separately results in cost penalties due to replication of the housing and sensor interface circuitry.